1. Field of the Invention
The invention relates to safety valves and, in particular, to safety valves for heating systems which are automatically cutoff when a hazardous temperature is sensed.
2. Description of the Prior Art
The prior art, as exemplified in U.S. Pat. Nos. 2,245,773, 2,289,287, 2,292,830, and 2,631,656, contains many heating systems employing recyclic safety switch cutoffs in series with a fuel control valve. The switches are mounted in the bonnet of the heating system for sensing an overheat condition; however, such recyclic safety switches can become defective or be shunted allowing the heating system to reach a dangerous heated condition.
A number of fluid expansion responsive valves in the prior art, as exemplified by U.S. Pat. Nos. 2,055,133, 2,627,911, 2,787,130, and 3,386,065, have been employed in many applications other than as safety cutoffs in a heating system; such valves having a number of deficiencies rendering them incapable of performing safety cutoff functions.
The prior art, as exemplified by U.S. Pat. No. 3,284,600, contains several liquid and gas charged switch devices charged at a partial vacuum to allow external atmospheric pressure to deflect a diaphragm to close a switch such that when the internal vapor or gas pressure equals atmospheric pressure, either due to a leak or an increase in temperature, the diaphragm is allowed to move to open the switch; such switch devices, when temperature operated, reclosing upon the temperature returning again to below the switch operating temperature.
Also, the prior art, as exemplified in U.S. Pat. Nos. 1,744,735, 3,258,363, 3,441,819, 3,516,791, and the publication (USSR Academy of Sciences, M. M. Dubinin, "Thermal Treatment and Microporous Structure of Carbonaceous Adsorbents", Proceedings of the Fifth Conference on Carbon, Vol. 1, 1962, pages 81-87) contains many adsorbent carbon materials including decomposed polyvinylidene chloride and polyvinylidene flouride. Adsorbent carbon materials are widely used in removing contaminants, or the like, from gases or liquids. Polyvinylidene chloride and polyvinylidene flouride, in particular, have been recognized for their molecular sieve property; that is, their ability to adsorb certain gaseous materials which have small molecular sizes while being incapable of adsorbing other gaseous materials which have larger molecular sizes.
U.S. Pat. Nos. 3,318,358, 3,495,925, and 3,726,630 disclose prior art electrical ignition circuits which can be employed in heating systems.